Press-formed members of automobiles and others have various shapes. In press forming for these members, plural forming elements are generally combined with each other, examples of the elements include deep drawing, bulging, stretch flanging, and bending. A member difficult to press-form, out of these members, is, for example, a member such as a door inner illustrated in FIG. 8, which has, in the bottom of its body, bulged regions A in a convex or convex form. About this member, the bulged regions A are formed by bulging at the late stage of deep drawing therefor. Examples of a press-formed member of such a type include, besides door inners, door outers, front pillars, center pillars, rear floors, and side sills. Deep drawing is a method of causing a material to flow into a die so as to be formed, and bulging is a method of extending a material in a die so as to be formed.
Usually, in press factories for producing these members, press forming is performed at a high forming velocity of 10 mm/sec or more to ensure high productivity. In press factories for automobile members, which pursue high productivity, press forming is performed at a high forming velocity of about 70 mm/sec in many cases. The forming velocity referred to herein is an average forming velocity during a period from a time when a punch contacts a blank so as to start an actual forming of the blank to an end of the forming.
In the automobile field in recent years, in order to improve automobiles in mileage to reduce the discharge of carbon dioxide, attempts to use high tensile steel plates for their press-formed members have been positively advanced for lightening their bodies. For a part of press-formed members, high tensile steel plates having a tensile strength of 980 MPa or higher have come to be used.
It is well known that as a steel plate is increased in strength, the ductility thereof is decreased. The press formability thereof is also decreased. Thus, in order that steel plates higher in strength can be applied to wider-spreading press-formed members, from the viewpoint of the material thereof, developments have been advanced about high tensile steel plates good in balance between strength and ductility. From the viewpoint of the working technique thereof, developments have been advanced about a press forming method for improving the limit of press forming.
Example of a high tensile steel plate good in balance between strength and ductility that has been so far developed include DP (dual phase) steel plates, which are composed of a ferrite phase and a martensite phase, and TRIP (transformation induced plasticity) type steel plates having retained austenite transformation induced plasticity (see, for example, Non Patent Literature 1). Recently, as a high tensile steel plate better in balance between strength and ductility, developments have been made also about a TBF (trip aided bainitic ferrite) steel plate of a TRIP type, which has bainitic ferrite as a parent phase (see, for example, Non Patent Literature 2).
As the press forming method for improving the limit of press forming, suggested are a method of press-forming a steel plate under conditions that the steel plate temperature at its punch region is set to normal temperature or lower and the steel plate temperature at its crease press region to 150° C. or higher (see, for example, Patent Literature 1), and a method of press-forming a TRIP type steel plate under conditions that the temperature of a mold at its die shoulder region is set into the range of 150 to 200° C., and that of the mold at its punch shoulder region into the range of −30 to 0° C. (see, for example, Patent Literature 2). In each of the methods described in Patent Literatures 1 and 2, deep drawing is performed to verify an advantageous effect of improving the deep drawing limit on the basis of local warm forming at a crease pressing region or die shoulder region.
Reports are also made about test results that individual tests were made in which TBF steel plates were used to examine the effect of temperature for forming the steel plates onto the press formabilities (bulgability, deep drawability and stretch flangability) to find out that there is a warm temperature range in which the bulgability, deep drawability and stretch flangability are made better than those in any cold temperature range (see, for example, Non Patent Literature 3). In the formation described in Non Patent Literature 3, the bulging tests and stretch flanging tests were made at a considerably lower forming velocity, 1 mm/min (0.017 mm/sec), than forming velocities in actual press factories, about 70 mm/sec. The deep drawing tests were made at a forming velocity of 200 mm/min (3.3 mm/sec).